Lateral Flow Assay Biotesting by Utilizing Plasmonic Nanoparticles Made of Inexpensive Metals─Replacing Colloidal Gold.

Nanoparticles (NPs) can be conjugated with diverse biomolecules and employed in biosensing to detect target analytes in biological samples. This proven concept was primarily used during the COVID-19 pandemic with gold-NP-based lateral flow assays (LFAs). Considering the gold price and its worldwide depletion, here we show that novel plasmonic NPs based on inexpensive metals, titanium nitride (TiN) and copper covered with a gold shell (Cu@Au), perform comparable to or even better than gold nanoparticles. After conjugation, these novel nanoparticles provided high figures of merit for LFA testing, such as high signals and specificity and robust naked-eye signal recognition. Since the main cost of Au NPs in commercial testing kits is the colloidal synthesis, our development with the Cu@Au and the laser-ablation-fabricated TiN NPs is exciting, offering potentially inexpensive plasmonic nanomaterials for various bioapplications. Moreover, our machine learning study showed that biodetection with TiN is more accurate than that with Au.

Reshaping and induction of optical activity in gold@silver nanocuboids by chiral glutathione molecules.

Core-shell gold-silver cuboidal nanoparticles were produced, with either concave or straight facets. Their incubation with a low concentration of chiral l-glutathione (GSH) biomolecules was found to produce near UV plasmonic extinction and induced circular dichroism (CD) peaks. The effect is sensitive to the silver shell thickness. The GSH molecules were found to cause redistribution of silver in the shell, removing silver atoms from edges/corners and re-depositing them at the nanocuboid facets, probably through some redox and complexation processes between the silver and thiol group of the GSH. Other thiolated chiral biomolecules (and drug molecules) did not show this effect. The emerging near UV surface plasmon resonance is a silver slab resonance, which might also possess some multipolar resonance nature. The concave-shaped nanocuboids exhibited stronger induced plasmonic CD relative to the nanocuboids with straight facets.

Nonclassical Mechanism of Metal-Enhanced Photoluminescence of Quantum Dots.

Metal-enhanced photoluminescence is able to provide a robust signal even from a single emitter and is promising in applications in biosensors and optoelectronic devices. However, its realization with semiconductor nanocrystals (e.g., quantum dots, QDs) is not always straightforward due to the hidden and not fully described interactions between plasmonic nanoparticles and an emitter. Here, we demonstrate nonclassical enhancement (i.e., not a conventional electromagnetic mechanism) of the QD photoluminescence at nonplasmonic conditions and correlate it with the charge exchange processes in the system, particularly with high efficiency of the hot-hole generation in gold nanoparticles and the possibility of their transfer to QDs. The hole injection returns a QD from a charged nonemitting state caused by hole trapping by surface and/or interfacial traps into an uncharged emitting state, which leads to an increased photoluminescence intensity. These results open new insights into metal-enhanced photoluminescence, showing the importance of the QD surface states in this process.

Investigating Plasmonic Catalysis Kinetics on Hot-Spot Engineered Nanoantennae.

Strong hot-spots can facilitate photocatalytic reactions potentially providing effective solar-to-chemical energy conversion pathways. Although it is well-known that the local electromagnetic field in plasmonic nanocavities increases as the cavity size reduces, the influence of hot-spots on photocatalytic reactions remains elusive. Herein, we explored hot-spot dependent catalytic behaviors on a highly controlled platform with varying interparticle distances. Plasmon-meditated dehalogenation of 4-iodothiophenol was employed to observe time-resolved catalytic behaviors via in situ surface-enhanced Raman spectroscopy on dimers with 5, 10, 20, and 30 nm interparticle distances. As a result, we show that by reducing the gap from 20 to 10 nm, the reaction rate can be sped up more than 2 times. Further reduction in the interparticle distance did not improve reaction rate significantly although the maximum local-field was ∼2.3-fold stronger. Our combined experimental and theoretical study provides valuable insights in designing novel plasmonic photocatalytic platforms.

Unraveling the Chirality Transfer from Circularly Polarized Light to Single Plasmonic Nanoparticles.

Due to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon-assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single-particle and ensemble levels. Theoretical simulations, including hot-electron surface maps, reveal that the plasmon-induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon-induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light-matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others.

DNA-Assembled Chiral Satellite-Core Nanoparticle Superstructures: Two-State Chiral Interactions from Dynamic and Static Conformations.

A significant challenge exists in obtaining chiral nanostructures that are amenable to both solution-phase self-assembly and solid-phase preservation, which enable the observation of unveiled optical responses impacted by the dynamic or static conformation and the incident excitations. Here, to meet this demand, we employed DNA origami technology to create quasi-planar chiral satellite-core nanoparticle superstructures with an intermediate geometry between the monolayer and the double layer. We disentangled the complex chiral mechanisms, which include planar chirality, 3D chirality, and induced chirality transfer, through combined theoretical studies and thorough experimental measurements of both solution- and solid-phase samples. Two distinct states of optical responses were demonstrated by the dynamic and static conformations, involving a split or nonsplit circular dichroism (CD) line shape. More importantly, our study on chiral nanoparticle superstructures on a substrate featuring both a dominant 2D geometry and a defined 3D represents a great leap toward the realization of colloidal chiral metasurfaces.

Chiral Generation of Hot Carriers for Polarization-Sensitive Plasmonic Photocatalysis.

Mastering the manipulation of chirality at the nanoscale has long been a priority for chemists, physicists, and materials scientists, given its importance in the biochemical processes of the natural world and in the development of novel technologies. In this vein, the formation of novel metamaterials and sensing platforms resulting from the synergic combination of chirality and plasmonics has opened new avenues in nano-optics. Recently, the implementation of chiral plasmonic nanostructures in photocatalysis has been proposed theoretically as a means to drive polarization-dependent photochemistry. In the present work, we demonstrate that the use of inorganic nanometric chiral templates for the controlled assembly of Au and TiO2 nanoparticles leads to the formation of plasmon-based photocatalysts with polarization-dependent reactivity. The formation of plasmonic assemblies with chiroptical activities induces the asymmetric formation of hot electrons and holes generated via electromagnetic excitation, opening the door to novel photocatalytic and optoelectronic features. More precisely, we demonstrate that the reaction yield can be improved when the helicity of the circularly polarized light used to activate the plasmonic component matches the handedness of the chiral substrate. Our approach may enable new applications in the fields of chirality and photocatalysis, particularly toward plasmon-induced chiral photochemistry.

Mach-Zehnder interferometer based integrated-photonic acetone sensor approaching the sub-ppm level detection limit.

The detection of acetone in the gaseous form in exhaled breath using an integrated sensor can provide an effective tool for disease diagnostics as acetone is a marker for monitoring human metabolism. An on-chip acetone gas sensor based on the principle of Mach-Zehnder interferometer is proposed and demonstrated. The sensing arm of the device is activated with a composite film of polyethyleneimine and amido-graphene oxide as the gas-sensitive adsorption layer. The composite film demonstrates good selectivity to acetone gas, can be used repeatedly, and is stable in long-term use. Room temperature operation has been demonstrated for the sensor with high sensitivity under a 20 ppm acetone environment. The detection limit can reach 0.76 ppm, making it feasible to be used for the clinical diagnosis of diabetes and the prognosis of heart failure.

Chiral Photomelting of DNA-Nanocrystal Assemblies Utilizing Plasmonic Photoheating.

Chiral plasmonic nanostructures exhibit anomalously strong chiroptical signals and offer the possibility to realize asymmetric photophysical and photochemical processes controlled by circularly polarized light. Here, we use a chiral DNA-assembled nanorod pair as a model system for chiral plasmonic photomelting. We show that both the enantiomeric excess and consequent circular dichroism can be controlled with chiral light. The nonlinear chiroptical response of our plasmonic system results from the chiral photothermal effect leading to selective melting of the DNA linker strands. Our study describes both the single-complex and collective heating regimes, which should be treated with different models. The chiral asymmetry factors of the calculated photothermal and photomelting effects exceed the values typical for the chiral molecular photochemistry at least 10-fold. Our proposed mechanism can be used to develop chiral photoresponsive systems controllable with circularly polarized light.

Local Growth Mediated by Plasmonic Hot Carriers: Chirality from Achiral Nanocrystals Using Circularly Polarized Light.

Plasmonic nanocrystals and their assemblies are excellent tools to create functional systems, including systems with strong chiral optical responses. Here we study the possibility of growing chiral plasmonic nanocrystals from strictly nonchiral seeds of different types by using circularly polarized light as the chirality-inducing mechanism. We present a novel theoretical methodology that simulates realistic nonlinear and inhomogeneous photogrowth processes in plasmonic nanocrystals, mediated by the excitation of hot carriers that can drive surface chemistry. We show the strongly anisotropic and chiral growth of oriented nanocrystals with lowered symmetry, with the striking feature that such chiral growth can appear even for nanocrystals with subwavelength sizes. Furthermore, we show that the chiral growth of nanocrystals in solution is fundamentally challenging. This work explores new ways of growing monolithic chiral plasmonic nanostructures and can be useful for the development of plasmonic photocatalysis and fabrication technologies.

Unraveling the Complex Chirality Evolution in DNA-Assembled High-Order, Hybrid Chiroplasmonic Superstructures from Multi-Scale Chirality Mechanisms.

Hierarchical, chiral hybrid superstructures of chromophores and nanoparticles are expected to give rise to intriguing unveiled chiroptical responses originating from the complex chiral interactions among the components. Herein, DNA origami cavity that could self-assemble into one-dimensional (1D) DNA tubes was employed as a scaffold to accurately organize metal nanoparticles and chromophores. The chiral interactions were studied at the level of individual hybrid particles and their 1D hybrid superstructures. Complex chirality mechanisms involving global structural chirality, plasmon-induced circular dichroism (PICD) and exciton-coupled circular dichroism (ECCD) were disentangled. The multiplexed CD spectrum superposition revealed the chirality evolution at different length scales. These results can offer a model for boosting the theoretical understanding of classical-quantum hybrid systems, and would inspire the future design of optically-active substances across length scales.

Distance Dependence of Förster Resonance Energy Transfer Rates in 2D Perovskite Quantum Wells via Control of Organic Spacer Length.

Two-dimensional (2D) semiconductors are attractive candidates for a variety of optoelectronic applications owing to the unique electronic properties that arise from quantum confinement along a single dimension. Incorporating nonradiative mechanisms that enable directed migration of bound charge carriers, such as Förster resonance energy transfer (FRET), could boost device efficiencies provided that FRET rates outpace undesired relaxation pathways. However, predictive models for FRET between distinct 2D states are lacking, particularly with respect to the distance d between a donor and acceptor. We approach FRET in systems with binary mixtures of donor and acceptor 2D perovskite quantum wells (PQWs), and we synthetically tune distances between donor and acceptor by varying alkylammonium spacer cation lengths. FRET rates are monitored using transient absorption spectroscopy and ultrafast photoluminescence, revealing rapid picosecond lifetimes that scale with spacer cation length. We theoretically model these binary mixtures of PQWs, describing the emitters as classical oscillating dipoles. We find agreement with our empirical lifetimes and then determine the effects of lateral extent and layer thickness, establishing fundamental principles for FRET in 2D materials.

Broadband thin-film and metamaterial absorbers using refractory vanadium nitride and their thermal stability.

Strong absorption of the full spectrum of sunlight at high temperatures is desired for photothermal devices and thermophotovoltaics. Here, we experimentally demonstrate a thin-film broadband absorber consisting of a vanadium nitride (VN) film and a SiO2 anti-reflective layer. Owing to the intrinsic high loss of VN, the fabricated absorber exhibits high absorption over 90% in the wide range of 400-1360 nm. To further enhance the near-infrared absorption, we also propose a metamaterial absorber by depositing patterned VN square patches on the thin-film absorber. An average absorption of 90.4% over the range of 400-2500 nm is achieved due to the excitation of broad electric dipole resonance. Both thin-film and metamaterial absorbers are demonstrated to possess excellent incident angle tolerances (up to 60°) and superior thermal stability at 800 ℃. The proposed refractory VN absorbers may be potentially used for solar energy harvesting, thermal emission, and photodetection.

Hot Electrons Generated in Chiral Plasmonic Nanocrystals as a Mechanism for Surface Photochemistry and Chiral Growth.

The realization of chiral photochemical reactions at the molecular level has proven to be a challenging task, with invariably low efficiencies originating from very small optical circular dichroism signals. On the contrary, colloidal nanocrystals offer a very large differential response to circularly polarized light when designed with chiral geometries. We propose taking advantage of this capability, introducing a novel mechanism driving surface photochemistry in a chiral nanocrystal. Plasmonic nanocrystals exhibit anomalously large asymmetry factors in optical circular dichroism (CD), and the related hot-electron generation shows in turn a very strong asymmetry, serving as a mechanism for chiral growth. Through theoretical modeling, we show that chiral plasmonic nanocrystals can enable chiral surface growth based on the generation of energetic (hot) electrons. Using simple and realistic phenomenological models, we illustrate how this kind of surface photochemistry can be observed experimentally. The proposed mechanism is efficient if it operates on an already strongly chiral nanocrystal, whereas our proposed mechanism does not show chiral growth for initially nonchiral structures in a solution. The asymmetry factors for the chiral effects, driven by hot electrons, exceed the values observed in chiral molecular photophysics at least 10-fold. The proposed chiral-growth mechanism for the transformation of plasmonic colloids is fundamentally different to the traditional schemes of chiral photochemistry at the molecular level.

Chiral Plasmonic Nanostructures Enabled by Bottom-Up Approaches.

We present a comprehensive review of recent developments in the field of chiral plasmonics. Significant advances have been made recently in understanding the working principles of chiral plasmonic structures. With advances in micro- and nanofabrication techniques, a variety of chiral plasmonic nanostructures have been experimentally realized; these tailored chiroptical properties vastly outperform those of their molecular counterparts. We focus on chiral plasmonic nanostructures created using bottom-up approaches, which not only allow for rational design and fabrication but most intriguingly in many cases also enable dynamic manipulation and tuning of chiroptical responses. We first discuss plasmon-induced chirality, resulting from the interaction of chiral molecules with plasmonic excitations. Subsequently, we discuss intrinsically chiral colloids, which give rise to optical chirality owing to their chiral shapes. Finally, we discuss plasmonic chirality, achieved by arranging achiral plasmonic particles into handed configurations on static or active templates. Chiral plasmonic nanostructures are very promising candidates for real-life applications owing to their significantly larger optical chirality than natural molecules. In addition, chiral plasmonic nanostructures offer engineerable and dynamic chiroptical responses, which are formidable to achieve in molecular systems. We thus anticipate that the field of chiral plasmonics will attract further widespread attention in applications ranging from enantioselective analysis to chiral sensing, structural determination, and in situ ultrasensitive detection of multiple disease biomarkers, as well as optical monitoring of transmembrane transport and intracellular metabolism.

Special topic on emerging directions in plasmonics.

Plasmonics enables a wealth of applications, including photocatalysis, photoelectrochemistry, photothermal heating, optoelectronic devices, and biological and chemical sensing, while encompassing a broad range of materials, including coinage metals, doped semiconductors, metamaterials, 2D materials, bioconjugates, and chiral assemblies. Applications in plasmonics benefit from the large local electromagnetic field enhancements generated by plasmon excitation, as well as the products of plasmon decay, including photons, hot charge carriers, and heat. This special topic highlights recent work in both theory and experiment that advance our fundamental understanding of plasmon excitation and decay mechanisms, showcase new applications enabled by plasmon excitation, and highlight emerging classes of materials that support plasmon excitation.

Chiral Plasmonic Nanocrystals for Generation of Hot Electrons: Toward Polarization-Sensitive Photochemistry.

The use of biomaterials, with techniques such as DNA-directed assembly or biodirected synthesis, can surpass top-down fabrication techniques in creating plasmonic superstructures in terms of spatial resolution, range of functionality, and fabrication speed. In particular, by enabling a very precise placement of nanoparticles in a bioassembled complex or through the controlled biodirected shaping of single nanoparticles, plasmonic nanocrystals can show remarkably strong circular dichroism (CD) signals. We show that chiral bioplasmonic assemblies and single nanocrystals can enable polarization-sensitive photochemistry based on the generation of energetic (hot) electrons. It is now established that hot plasmonic electrons can induce surface photochemistry or even reshape plasmonic nanocrystals. We show that merging chiral plasmonic nanocrystal systems and the hot-election generation effect offers unique possibilities in photochemistry, such as polarization-sensitive photochemistry promoting nonchiral molecular reactions, chiral photoinduced growth of a colloid at the atomic level, and chiral photochemical destruction of chiral nanocrystals. In contrast, for chiral molecular systems, the equivalent of the described effects is challenging to observe because molecular species typically exhibit very small CD signals. Moreover, we compare our findings with traditional chiral photochemistry at the molecular level, identifying new, different regimes for chiral photochemistry with possibilities that are unique for plasmonic colloidal systems. In this study, we bring together the concept of hot-electron generation and the field of chiral colloidal plasmonics. Using chiral plasmonic nanorod complexes as a model system, we demonstrate remarkably strong CD in both optical extinction and generation rates of hot electrons. Studying the regime of steady-state excitation, we discuss the influence of geometrical and material parameters on the chiral effects involved in the generation of hot electrons. Optical chirality and the chiral hot-electron response in the nanorod dimers result from complex interparticle interactions, which can appear in the weak coupling regime or in the form of Rabi splitting. Regarding practical applications, our study suggests interesting opportunities in polarization-sensitive photochemistry, in chiral recognition or separation, and in promoting chiral crystal growth at the nanoscale.

DNA-Enabled Chiral Gold Nanoparticle-Chromophore Hybrid Structure with Resonant Plasmon-Exciton Coupling Gives Unusual and Strong Circular Dichroism.

Circular dichroism (CD) from hybrid complexes of plasmonic nanostructures and chiral molecules has recently attracted significant interest. However, the hierarchical chiral self-assembly of molecules on surfaces of metal nanostructures has remained challenging. As a result, a deep understanding of plasmon-exciton coupling between surface plasmons and chiral collective molecular excitations has not been achieved. In particular, the critical impact of resonant plasmon-exciton coupling within the hybrid is unclear. Here, we employed DNA-templated strategies to control the chiral self-assembly of achiral chromophores with rationally tuned exciton transitions on gold nanosphere (AuNP) or gold nanorod (AuNR) surfaces. Unlike many previous chiral plasmonic hybrids utilizing chiral biomolecules with CD signals in the UV range, we designed structures with the chiral excitonic resonances at visible wavelengths. The constructed hybrid complexes displayed strong chiroptical activity that depends on the spectral overlap between the chiral collective molecular excitations and the plasmon resonances. We find that when spectral overlap is optimized, the molecular CD signal originating from the chiral self-assemblies of chromophores was strongly enhanced (maximum enhancement of nearly an order of magnitude) and a plasmonic CD signal was induced. Surprisingly, the sign of the molecular CD was reversed despite different self-assembly mechanisms of the Au nanoparticle-chromophore hybrids. Our results provide new insight into plasmonic CD enhancements and will inspire further studies on chiral light-matter interactions in strongly coupled plasmonic-excitonic systems.

Generation of hot electrons in nanostructures incorporating conventional and unconventional plasmonic materials.

The generation of energetic electrons is an effect occurring in any plasmonic nanostructure. However, the number of electrons with high energies generated optically in a plasmonic nanostructure can be relatively small. This is an intrinsic property of the collective plasmon excitations in a Fermi gas of electrons. But the choices of material and geometry have a great impact on the generation rate, and are therefore crucial for designing a nanostructure with a large rate of generation of energetic (hot) electrons. Here we test different plasmonic materials from the point of view of the generation of hot electrons (HEs). Our choice of materials includes both strongly-plasmonic materials (Au, Ag, Cu and Al) and crystals with strongly broadened plasmonic resonances (Pt, TiN and ZrN). Regarding the choice of geometry, we consider two types of nanostructures, single nanocrystals deposited over a dielectric substrate and metastructure absorbers, observing interesting opto-electronic properties. For single nanocrystals, the rate of HE generation is strongly material-dependent since the HE generation rate strongly depends on several physical parameters such as plasmonic enhancement, plasmonic resonance wavelength, Fermi energy, etc. Interestingly, the plasmonic meta-absorbers exhibit a different behaviour. The strongly-plasmonic metals, such as Au, Ag, Cu or Al, show very similar performances, while the materials with damped plasmon resonances demonstrate diverse and reduced rates of HE generation. The physical reason for these different behaviours lies in the dielectric functions of these materials. In the metastructures, plasmonic resonances are in the infrared region and the strongly-plasmonic materials behave as an almost ideal metal, whereas the second group of materials exhibits strong dissipation. This makes the responses from the metastructures made of crystals with damped plasmons strongly dependent on the choice of material. The physical principles described in our study can be useful for designing metastructures and nanodevices based on HEs, which can be used in photo-chemistry and opto-electronics.

Gap-plasmon enhanced water splitting with ultrathin hematite films: the role of plasmonic-based light trapping and hot electrons.

Hydrogen is a promising alternative renewable fuel for meeting the growing energy demands of the world. Over the past few decades, photoelectrochemical water splitting has been widely studied as a viable technology for the production of hydrogen utilizing solar energy. A solar-to-hydrogen (STH) efficiency of 10% is considered to be sufficient for practical applications. Amongst the wide class of semiconductors that have been studied for their application in solar water splitting, iron oxide (α-Fe2O3), or hematite, is one of the more promising candidate materials, with a theoretical STH efficiency of 15%. In this work, we show experimentally that by utilizing gold nanostructures that support gap-plasmon resonances together with a hematite layer, we can increase the water oxidation photocurrent by two times over that demonstrated by a bare hematite film at wavelengths above the hematite bandgap. Moreover, we achieve a six-fold increase in the oxidation photocurrent at near-infrared wavelengths, which is attributed to hot electron generation and decay in the gap-plasmon nanostructures. Theoretical simulations confirmed that the metamaterial geometry with gap plasmons that was used allows us to confine electromagnetic fields inside the hematite semiconductor and to enhance the surface photochemistry.